1. Field of the Invention
The present invention relates to conductive alloy powder and a dispersion-type conductor produced by dispersing the alloy powder in a binder, and is applicable to electromagnetic-wave shielding materials, membrane switches, circuit substrates and electronic parts.
2. Description of the Related Art
Conventionally, conductive powders such as carbon black powder, noble metal powder formed of, such materials as gold, silver and palladium, base metal powder formed of, such materials as copper and nickel, and alloys consisting of these metals are known. Dispersion-type conductors produced by dispersing such conductive powder in organic or inorganic binders are also known. Further, a method of producing such a known dispersion-type conductor is generally as follows. The conductive powder and a binder are kneaded, and a solvent and an additive are added thereto if necessary, so as to produce paste or paint. Such paste or paint is applied by a known method, such as a screen printing, and then thermally treated and solidified. As a result, a coat of the dispersion-type conductor is formed.
Carbon black represented by the conventional conductive powder for a dispersion-type conductor has low conductivity so that a dispersion-type conductor having low resistance cannot be obtained by using such powder. Also, since the base metal powder such as copper, nickel, or the like is likely to be oxidized on the surface, the electric resistance is raised in the contact area among the grains of the metal powder, and thus the resistivity of the dispersion-type conductor using such metal powder cannot be lowered as desired although the electric resistance inherent in the metal is low. Further, noble metal such as gold, silver, platinum, palladium, or the like, and the powder of these alloys are less likely to be oxidized on the surface, and thus the use of such powders for the dispersion-type conductor makes it possible to lower the resistivity of the conductor. However, since the noble metal is expensive, the dispersion-type conductor is also costly. In general, silver powder, which metal is less costly than other noble metals, is currently used for the dispersion-type conductor. However, if voltage is applied to such dispersion-type conductor when water is present between a pair of conductors separate from each other, the silver is ionized in a dispersion-type conductor having a positive potential by an electrochemical reaction, and is transferred to another conductor having a negative potential and is precipitated therein; what is called, electromigration, is likely to occur. If the electromigration continues for quite a while, the precipitated silver expands from being a conductor having a negative potential to a conductor having a positive potential, which deteriorates the insulation between the conductors and even brings about short-circuiting thereof.
Recently, silver alloy powder obtained by employing a high-pressure atomizing method and a conductor using such silver alloy powder have been proposed with a view to overcoming the above-mentioned drawbacks of the conductive powder. The details of such silver alloy powder and a conductor using the same are disclosed in Japanese Patent Laid-Open Nos. 3-245404 and 3-245405. A common object of these inventions is to provide silver alloy powder and a conductive paste using such powder which are excellent in characteristics such as conductivity, oxidation resistance, and anti-migration of silver. An object of the invention disclosed in Japanese Patent Laid-Open No. 3-245404 is further to provide conductive paste which is low in price. These inventions are provided on the basis of a conventional theory of combining silver and a base metal by utilizing the advantages of both metals effectively and compensating for the shortcomings thereof. Also, one of the features of these inventions is that metal powder is transformed into an alloy by employing a high-pressure atomizing method, thereby obtaining a remarkable effect. The high-pressure atomizing method is employed by the following process. Melt having a desired composition is ejected into an inert gas atmosphere, and concurrently, high-speed gas generated by the method of adiabatic-expansion of highly-pressurized inert gas is ejected on the melt so as to quench and solidify the melt. A description is not fully given in such patent publications of what sort of characteristics will arise within the alloy by employing the high-pressure atomizing method. However, Japanese Patent Laid-Open No. 3-245404 discloses that the silver concentration is gradually increased toward the surface of the powder in a particular region. It also proves that this is because a liquid phase full of a silver component having a low melting point appears on the surface of the powder in the process of solidifying the powder by quenching, and such phase is delayed from being solidified in comparison to other regions, which results in the silver concentration on the surface.
The alloy powder produced by employing the high-pressure atomizing method has better characteristics than the conductive powder theretofore, but it is not yet industrially satisfactory to a sufficient extent.